1. Field of the Invention
The present invention relates to a light-emitting device composed of a light-emitting diode, and particularly to a light-emitting device composed of a light-emitting diode chip having an anode electrode and a cathode electrode both arranged on the top surface thereof.
2. Description of the Prior Art
A typical light-emitting diode (LED) that emits light of a relatively long wavelength, such as red or green light, has one electrode on its top surface and another electrode on its bottom surface. A light-emitting diode chip of this type is usually formed into a light-emitting device through a process as described below. First, the LED chip is fixed on a base board with adhesive. This step is called die bonding and, here, a conductive adhesive is used so that, at the same time that the LED chip is fixed on the base board, its bottom-surface electrode will be connected to a metal conductor laid on the base board. Next, the top-surface electrode of the LED chip is connected, by way of a wire of a highly conductive material such as gold, to another metal conductor laid on the base board. This step is called wire bonding. Last, the LED chip thus fixed on the base board is, together with the wire, sealed in a mold of light-permeable resin for protection.
In wire bonding, a tool having the shape of a small-diameter cylinder, called a capillary tool, is used. First, the capillary tool is moved down to the top-surface electrode of the LED chip in such a way that a ball-shaped tip of a wire that sticks out from the lower tip of the capillary tool is pressed against the electrode with a pressure not so strong as to damage the LED chip. Then, from the capillary tool, an ultrasonic wave is applied to the tip of the wire, and thereby the wire is fixed on the electrode. Next, the capillary tool is moved sideways so that it can then be moved down to the corresponding metal conductor formed on the base board, where, by similar application of a pressure and an ultrasonic wave, the wire is connected to the metal conductor and is then cut.
In a light-emitting device manufactured in this way, the LED chip protrudes from the top surface of the base board, and is thus susceptible to an external mechanical force. In particular, a strong mechanical force applied from the side may cause the LED chip, together with the resin mold that is supposed to provide protection thereto, to separate from the base board. It is also possible that the wire is broken midway, or the wire separates, at one or the other end thereof, from the electrode formed on the chip or from the conductor formed on the circuit board. Thus, in conventional light-emitting devices, an external mechanical force often causes imperfect contact.
Japanese Laid-Open Patent Application No. H5-29650 discloses a photointerruptor that has a light-emitting chip and a light-sensing chip placed in a cavity formed in its case, with the cavity filled with light-permeable resin. The cavity has metal conductors laid on its floor surface, and the two chips are, within this cavity, connected individually to those metal conductors in the manner as described above. Specifically, each chip is fixed, by die bonding, on the floor surface of the cavity with conductive adhesive, and thereby the electrode placed on the bottom surface of the chip is connected to a conductor laid on the floor surface of the cavity; on the other hand, the electrode placed on the top surface of the chip is connected, by wire bonding, to another conductor laid on the floor surface of the cavity.
In this photointerruptor, neither the chip nor the wires protrude from the case, and therefore an external mechanical force is unlikely to cause imperfect contact. However, those wires, which are used to achieve electrical connection within the cavity formed in the case, need to be fixed somewhere within the cavity, and, for this purpose, it is necessary to secure extra space beside the chip within the cavity. In addition, to prevent the capillary tool from touching the wall surface of the cavity, it is necessary to make the cavity sufficiently large. These requirements inevitably make the photointerruptor as a whole unnecessarily large.
Japanese Laid-Open Patent Application No. H7-15046 discloses a light-emitting device in which a cavity whose depth is greater than the height of the LED chip is formed in the top surface of the base board so that the LED chip will be placed within that cavity. The base board has two metal conductors laid on its top surface, and one of those metal conductors is so formed as to extend to and then over the floor surface of the cavity. The LED chip is fixed, by die bonding, on the floor surface of the cavity with conductive adhesive, and thereby the bottom-surface electrode of the chip is connected to the conductor that reaches the floor surface of the cavity. On the other hand, the top-surface electrode of the chip is connected, by wire bonding, to the conductor that remains on the top surface of the base board.
In this light-emitting device, the conductor on which the wire is fixed is laid on the top surface of the base board, in contrast to the above-described photo-interruptor in which the corresponding conductor is laid on the floor surface of the cavity. This eliminates the need to secure extra wire-fixing space within the cavity, and thus makes it possible to make the cavity accordingly smaller. Instead, however, the top surface of the LED chip is kept lower than the top surface of the base board. As a result, when one end of the wire is fixed on a electrode formed on the top surface of the chip, it is possible that the lower tip of the capillary tool touches the wall surface of the cavity or the top surface of the base board around the cavity. This can be avoided by making the cavity larger, but this inevitably makes the light-emitting device as a whole unnecessarily large.
In recent years, LEDs that emit light of a relatively short wavelength, such as blue light, have been developed. Such LEDs are produced by laminating layers of gallium-nitride-based semiconductor over the surface of an insulating substrate of sapphire, and their structure requires that both an anode electrode and a cathode electrode be arranged on the top surface of an LED chip.
As disclosed in Japanese Laid-Open Patent Application No. H6-120562 for example, an LED chip of this type, which has an anode electrode and a cathode electrode both arranged on its top surface, is, when formed into a light-emitting device for practical use, fixed on a leadframe upside down and then sealed in a mold of light-permeating resin with lead terminals sticking out therefrom. FIG. 5 illustrates the light-emitting device of this Japanese Laid-Open Patent Application.
As shown in FIG. 5, this light-emitting device has an LED chip 50, which is composed of a sapphire substrate 51, an n-type GaN layer 52, a p-type GaN layer 53, an anode electrode 54, and a cathode electrode 55. The anode and cathode electrodes 54 and 55 are individually fixed on lead frames 57 with conductive adhesive 56. The entire LED chip 50 and the base portions of the lead frames 57 are sealed in a mold 58 of epoxy resin, with the tip portions of the lead frames 57 sticking out from the resin mold 58 so as to serve as lead terminals. The resin mold 58 is so shaped as to have a spherical surface at its top end so that the light emitted by the LED chip 50 will be condensed.
In practical use, this light-emitting device is mounted on the top surface of a circuit board, such as has through holes formed therein and has a conductive pattern formed on the bottom surface thereof, with the lead terminals placed through appropriate ones of those through holes. The lead terminals are then soldered, on the bottom surface of the circuit board, to the conductive pattern, and thereby the light-emitting device is fixed on the circuit board and connected to the conductive pattern at the same time. The light emitted by the LED chip 50 is exploited from the top-surface side of the circuit board.
However, in this light-emitting device, the junction surface between the semiconductor layers 52 and 53, where light emission occurs, is located below the sapphire substrate 51, and therefore it is inevitable that the emitted light is partially absorbed and deflected by the sapphire substrate 51. Thus, it is impossible to make full use of the emitted light. Moreover, it is also inevitable that this light-emitting device has a greater thickness than a conventional light-emitting device of the type that has a chip fixed simply on a base board. Furthermore, this light-emitting device is unfit for surface mounting, in which a conductive pattern is formed on the top surface of a circuit board and the light-emitting device is, on the top surface of the circuit board, fixed on the circuit board and connected to the conductive pattern by reflow soldering or a similar method.
Even an LED chip that has both electrodes arranged on its top surface can be formed into a light-emitting device that has the chip fixed simply on a base board. In that case, the LED chip is fixed, by die bonding, on the base board, and the two electrodes arranged on the top surface of the chip are individually connected, by wire bonding, to two metal conductors laid on the base board. Then, the chip, together with the two wires, is sealed in a resin mold for protection. In this way, it is possible to obtain a light-emitting device that has a structure similar to that of conventional light-emitting devices. In this light-emitting device, the emitted light is exploited directly from above, and thus it is possible to avoid the absorption or deflection of the light caused by the sapphire substrate. Moreover, this light-emitting device can be adapted for surface mounting by laying the metal conductors in such a way that they reach the bottom surface of the base board.
However, as described earlier, this structure is prone to imperfect contact caused by an external mechanical force. This can be avoided by forming a cavity in the base board and placing the LED chip in that cavity, but then, whether the wires be connected to conductors laid on the floor surface of the cavity as in the above-mentioned Japanese Laid-Open Patent Application No. H5-29650, or the wires be connected to conductors laid on the top surface of the base board that is situated at a level higher than the top surface of the chip as in the above-mentioned Japanese Laid-Open Patent Application No. H7-15046, it is necessary to make the cavity sufficiently large. This inevitably makes the light-emitting device as a whole unnecessarily large. In particular, having the structure in which the two electrodes individually require wire bonding, the light-emitting device needs to be made even larger than those with an LED chip having one electrode on its top surface and the other on its bottom surface.